dev tools

WIP

• devtool configuration option
• webpack-dev-server
• webpack-dev-middleware
• koa-webpack - Development and Hot Module Reload Middleware for Koa2. Simple setup and use. Composes webpack-dev-middleware and webpack-hot-middleware.
• koa-webpack-dev - serve bundle + Hot Module Replacement in Koa.js development server
• webpack-build-notifier - plugin to display OS-level notifications for Webpack build errors and warnings.